1. Field of the Invention
This invention relates to a DC motor drive circuit, and more particularly to a DC motor drive circuit which prevents commutation failure in a permanent magnet-type DC motor driven by a thyristor-controlled three-phase full-wave reversible bridge system.
2. Description of the Prior Art
There is at present a demand for a drive circuit which can drive a permanent magnet-type DC motor at a high efficiency by causing armature current to decrease with respect to demand output, this decrease in armature current being accomplished by driving the motor so as to generate a high armature voltage.
The speed of a DC motor is varied by changing the applied DC voltage, and reversing the polarity of the applied DC voltage causes the motor reverse rotation. In conventional practice, a thyristor-controlled three-phase full-wave reversible bridge system is employed to drive a DC motor which is required to be rotated at any optional speed and in both the forward and reverse directions, the bridge system accomplishing this merely by controlling the applied voltage and its directionality. The bridge system is of the type composed of a forward thyristor-controlled bridge for supplying the DC motor armature with a forward current, and a reverse thyristor-controlled bridge for supplying the armature with a reverse current, the two bridge system being connected in parallel with each other and in parallel with the armature. Six thyristors are used to construct each bridge system. The DC motor can be made to rotate in the forward direction by firing each thyristor in the forward thyristor-controlled bridge at a suitable timing to allow the forward bridge to supply the motor armature with a forward current, whereas the motor is stopped by regenerative braking and then made to rotate in the reverse direction by firing each thyristor in the reverse thyristor-controlled bridge at a suitable timing to permit the reverse bridge to supply the motor armature with a reverse current. In addition, controlling the firing angle of each thyristor varies the voltage impressed upon the armature and thus permits variation of motor speed. This is referred to as a static Ward-Leonard speed control system.
In general, speed control of a DC motor using the abovesaid bridge proceeds in the following manner. In brief, the deviation between the actual speed and a command speed is computed, and the firing angle of the thyristors is controlled to reduce this deviation to zero. More specifically, the firing angle of the thyristors that construct the forward thyristor-controlled bridge is advanced or retarded in accordance with the value of the speed deviation if the command signal tends toward the forward direction, and similarly the firing angle of the thyristors in the reverse thyristor-controlled bridge is advanced or retarded in accordance with the value of the speed deviation if the command signal tends toward the reverse direction. This control operation varies the voltage impressed upon the DC motor to raise or lower its speed, the motor thus being controlled at 11 times so as to rotate according to a prescribed speed deviation derived from the command speed.
On the other hand, if under this speed control the command speed is to be changed abruptly with the DC motor rotating in the forward direction according to the prescribed speed deviation from the command speed, the speed control operation proceeds in the following manner. It will first be assumed that the command speed is reduced in an abrupt manner. The operating mode of the foregoing bridge system prior to the change in the command speed is the power rectification mode in which the forward thyristor-controlled bridge delivers a forward current to the armature to effect the speed control as described above. After the reduction in the command speed the operating mode shifts to a power inversion mode in which a reverse current, i.e., a braking current, is fed back via the reverse thyristor-controlled bridge to the power source owing to the armature voltage, whereby the speed of the DC motor is quickly reduced until it coincides with the command speed. After the actual speed has reached the command speed the power rectification mode is restored. Thereafter, the firing angle of thyristors in the forward converter is controlled to rotate the DC motor, maintaining the deviation between the actual speed and command speed at a prescribed value.
The fact that the permanent magnet-type DC motor does not possess a field winding explains why its speed is reduced by the shift from the power rectification mode to the power inversion mode when the command speed is decreased. In other words, when seeking to reduce the speed of a DC motor having a field winding, a reverse armature current, namely a so-called braking current that is fed back to the power source, is caused to flow if the armature voltage is made higher than the applied AC voltage by increasing the field current which flows into the field winding. This causes the speed of the DC motor to decrease rapidly and is known as regenerative braking. It follows that a permanent magnet-type DC motor is reduced in speed by the reverse current (braking current) which flows into the armature due to the shift from the power rectification mode to power inversion mode at the time the command speed is decreased.
No particular problem arises in the changeover from the power rectification mode to the power inversion mode, but there is a commutation failure in the thyristors of the reverse thyristor-controlled bridge in the changeover from the power inversion mode to the power rectification mode. In particular, this commutation failure occurs when the armature voltage is higher than the AC supplied voltage, as in the case where the permanent magnet-type DC motor is rotated at high speed in order to drive it at a low current and a high efficiency. When thyristor commutation failure induces an excessive braking current in the armature, the commutator portion of the DC motor is damaged, and the permanent magnet is demagnetized. Moreover, the commutation failure does not allow a thyristor to be turned off and a short-circuit is caused. In this case an over-current accident such as a blown fuse occurs.